The commercially available full contour (monolithic) Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) dental ceramics have been used as a dental restorative material for over a decade for their superior mechanical properties in spite of being aesthetically inferior with lower translucency and lack of opalescence compared to lithium disilicate or leucite-based glass ceramic materials like IPS e.max or IPS Empress. While increasing the amount of doped yttria could effectively reduce the opacity of Y-TZP, it significantly lowers its mechanical strength and limits its clinical use for multi-unit restorations. The solution to this dilemma is to use nano-sized zirconia particles and thus to make zirconia restoration containing mean grain size around 100 nm or less. Such nanozirconia would not only yield higher mechanical strength than conventional zirconia but also maximize Y-TZP's translucency by reducing the size of scattering centers.
The commercialization of nanozirconia dental restorations requires the ability to produce a viable nanozirconia body of bulk size (thickness is 10 mm or greater) with uniform microstructure. Such nanozirconia restorations have not been reported so far in literature and the dental industry lacks the technology for bulk shape consolidation. The current state of art of processing zirconia is not suitable for nanozirconia for at least some of the following reasons:
1. Dry processing methods are not applicable. Due to the inherent high specific surface area of nanoparticles, they are prone to strong agglomeration. This strong propensity to agglomerate can result in undesirable material properties when processed using typical manufacturing methods for conventional zirconia such as die pressing.2. Most of liquid processing methods are not applicable. For example, slip casting of nanozirconia suspension is not able to produce thick bodies due to its low suction power associated with capillary force of molds. Direct coagulation casting would introduce inhomogeneities and requires high solid loading prior to casting stages. Gel casting methods use large amount of organics and later cause difficulty during debinding. Centrifugal consolidation from suspension is subject to non-uniformities of solid loading/relative density in the bulk because of the segregation of nanoparticles of different sizes.
Liquid processing methods from colloidal suspension are still a useful approach to produce viable dental articles despite two technical challenges to make mass manufacturing nanozirconia possible. First, to invent techniques of consolidating and drying bulk shapes with uniform/homogenous structure at the nano scale in each processing step; second, to invent techniques of processing bulk shape after casting, which include removing unwanted water and any processing agents during drying and debinding through extremely small pores/channels (pore size less than 10 nm) while maintaining the integrity of the nanozirconia bodies.